Quote:
Originally Posted by ct92404
The most confusing thing is that I don't see how it would even be possible to connect C16 the way it's shown in the diagram. If anyone has a chance to look at the schematic at the link I posted before, please look at page 110.
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Hi,
Don't be alarmed, what you are looking at is a very common configuration used in American television design. Admiral were using it as early as 1948.
A bright spark engineer figured out that the audio output stage, in its entirety, could be used as a DC voltage dropping device (ballast), to lower the voltage and provide power to other circuits in the set. This saved wasting power(heat) in a dropping resistor when a valve is already there doing it.
In Admiral sets, they used the voltage to power the entire IF stages. In your set they use it (the 140v) to power the video driver amplifier and looks like the tuner & ist IF amplifier. In any case current from the cathode of the audio output valve creates the 140V rail, that is where that 140V power rail comes from. But of course it needs the filter electrolytic caps (which are wired correctly) to remove any signals at audio frequencies. Clever isn't it ?
Just some words of advice when attempting to run a set of this vintage for the first time:
Never power a set this old up with any of the original electrolytic capacitors in it. If you do you will be chasing exploding capacitors and faults until the cows come home.
Document the wiring as best you can with diagrams and photographs and compare this with the schematics, sometimes you will find modifications.
Replace all of the electrolytic caps in the set before you do anything else.There is no such thing as a reliable 60+ year old electrolytic capacitor, despite what others may tell you.
Then check all the other capacitors as best you can, especially wax paper caps feeding valve grid circuits, replace if they have any detectable leakage at all. Replace any burnt looking or obviously out of spec resistors, only then power it up.
You will then have a good probability of no disasters/setbacks and a minimal number of faults to find after that basic work is done. Rather than being presented with a cocktail of difficulties due to multiple sub-circuit faults.
It is a great set you have there, well done for noticing it and wanting to repair it.
Hugo.